Apocalypse question, for those who have the sup....

I am going to piggy back onto this question and ask this as forgeworld has been zero help. In the Imperial armour book 3 and 4 and Apocalypse, are there a lot of the same rules in these books? Does it make it worth while to buy all 3? Also in book three which is the Tau one, since forgeworld no longer sells any kroot or half of their tank upgrades or flyers, are the rules still in there for all of these models that were once there?